In surgery, it is known to employ implants manufactured of biodegradable (under tissue conditions absorbable) polymers for connecting tissues together, and for securing the healing or growth of the tissues. It is known to manufacture of partly crystalline or amorphous thermoplastic, biodegradable polymers strong implant materials by stretching elongated blanks, such as bars, in a such manner that the structure of the materials is modified and directed (oriented) increasing the strength and the stiffness of the material in the direction of orientation.
U.S. Pat. No. 4,671,280 describes a tissue fastening device comprising (a) a fastener member having a pair of legs extending from the same side of a connecting cross piece, said fastener member adapted to be placed on one side of the tissue to be joined with the legs penetrating the tissues, said fastener member being an oriented crystalline polymeric material, whereby the fastener member has sufficient inherent strength and stiffness so that said legs can penetrate the tissue to be fastened, and (b) a receiver member to secure said fastener member in place. U.S. Pat. No. 4,671,280 describes the method to increase strength and stiffness of a tissue fastening device with solid state orientation (drawing the fastening device preform to the draw ratio of ca. 7 X).
U.S. Pat. No. 4,898,186 describes an osteosynthetic pin characterized in that the pin is formed substantially of a poly-L-lactic acid having a molecular weight of approximately at least 70,000, and is formed by axially drawing ca 2 to ca 10 times at an elevated temperature of ca 70° to ca 120° C. The osteosynthetic pin of U.S. Pat. No. 4,898,186 can be of any desired shape, for example, in the form of a plate, polygonal prism (such as quadrangular, pentagonal, hexagonal or octagonal prism), solid cylinder or the like. Polygonal prism structure may give for the pin a moderate rotational stability, but it needs an exact diameter of the drillhole in the bone, because polygonal prism structure cannot deform significantly if the drillhole is a bit too small in relation to the maximum transverse diameter of the pin.
U.S. Pat. No. 4,968,317 describes a surgical composite comprising a material selected from the group of resorbable polymer, resorbable copolymer, and mixtures thereof and further containing oriented, at least partially fibrillated structural units (fibrils) which have been induced into the material providing said units while said material is in its original nonfibrillar state by drawing said material in solid state. This patent describes also rods made of fibrillated material.
At higher drawing ratios (typically at drawing ratios of 7 to 10 or higher) (partial) fibrillation or self-reinforcing of material takes place, as is described in U.S. Pat. No. 4,968,317. (Partially) fibrillated materials have good strength properties in the direction of drawing, but these materials have poor strength properties in the direction perpendicular to the drawing direction, because longitudinal fibrils easily separate from each other. Therefore peeling of fibrous material from the surface of a pin, tack or mechanically machined screw may occur easily when a fibrillated (self-reinforced) pin, tack or screw is inserted into a drillhole in a bone.
U.S. Pat. No. 6,503,278 describes a material that degrades after implantation into patient's tissue, and resorbs into the patient's body, which material is manufactured of polymer, copolymer or polymer alloy. The material has a non-crystalline, i.e. amorphous structure and is molecularly oriented and reinforced by mechanical deformation. Further, the material can be formed into surgical devices, such as screws and pins, for implantation into a patient.
Pin with a solid cylinder structure needs a drillhole of specific size, because too large a drillhole gives a loose pin fixation and too small a drillhole prevents totally the insertion of a cylindrical pin. Additionally, the rotational stability of a bone fragment fixed with one smooth, cylindrical pin is poor.
U.S. Pat. No. 6,296,641 describes a substantially bioabsorbable, one-piece implant for tissue fixation comprising: an elongated cannulated shaft having a longitudinal axis and a first end, said first end having first and second sides, said first end comprising a mating surface projecting only from said first side of said elongated cannulated shaft so that said second side of said first end of said elongated cannulated shaft has a smooth surface without a projection, said mating surface being oriented at an angle of less than 90 degrees with respect to said longitudinal axis of said shaft, and said mating surface having a width in a direction perpendicular to the longitudinal axis that is greater than the width of said shaft. A fixation implant generally includes at least one shaft configured to securely fit into a hole formed in bone. The shaft is sufficiently long in relation to the interior diameter of the hole and, in some preferred embodiments, has locking protuberances, such as threads, ridges, or barbs, that resist the removal of the shaft from the hole in the bone when different types of forces, such as tensile or bending forces, are applied to the implant. The locking protuberances of implants of this invention are limited to transverse structures in relation to the longitudinal axis of the implant.